Hydrocarbons can be produced through a wellbore traversing a subterranean formation. The wellbore may be relatively complex. For example, the wellbore can include branch wellbores, such as multilateral wellbores and/or sidetrack wellbores. Multilateral wellbores include one or more lateral wellbores extending from a parent (or main) wellbore. A sidetrack wellbore is a wellbore that is diverted from a first general direction to a second general direction. A sidetrack wellbore can include a main wellbore in a first direction and a secondary wellbore diverted from the main wellbore and in a second general direction. A multilateral wellbore can include a window to allow lateral wellbores to be formed. A sidetrack wellbore can include a window to allow the wellbore to be diverted to the second general direction.
A window can be formed by positioning a casing joint and a whipstock in a casing string at a desired location in the main wellbore. The whipstock can deflect one or more mills laterally (or in one or more various orientations) relative to the casing string. The deflected mills penetrate part of the casing joint to form the window in the casing string through which drill bits can form the lateral wellbore or the secondary wellbore.
Casing joints are often made from high-strength material. The high-strength material may also be non-corrosive to withstand corrosive elements, such as hydrogen sulfide and carbon dioxide, which may be present in the subterranean environment. Milling a portion of the high-strength material can be difficult and can create a large amount of debris, such as small pieces of the casing joint, that can affect detrimentally well completion and hydrocarbon production. The debris can prevent the whipstock from being retrieved easily after milling is completed, plug flow control devices, damage seals, obstruct seal bores, and interfere with positioning components in the main bore below the casing joint.
Casing joints with pre-milled windows can be used to reduce or eliminate debris. The pre-milled windows can include an outer liner (or sleeve) to prevent particulate materials from entering the inner diameter of the casing string. The outer liner, which can be made from aluminum or fiberglass for example, can be milled easily and milling the outer liner can result in less debris as compared to drilling a window through a casing joint made from high-strength material. O-rings can be provided at each end of the outer sleeve to provide a seal between the outer sleeve and the casing joint.
The outer liners and the O-rings increase the outer diameter of the casing string. In some applications, the outer diameter may be increased by one or more inches. An increase in the outer diameter can be unacceptable in some situations.
Therefore, an assembly through which a window can be formed is desirable that can provide sufficient support for a casing string and avoid requiring an increase in the outer diameter of the casing string. An assembly that can avoid introducing an unacceptable amount of debris after the window is formed through milling is also desirable.